Year 7 Science · Unit 2 · Lesson 10
Challenge Worksheet
Learning Goals
Find the errors
A student wrote these revision notes. Three of the five lines contain one error each. For each line, write “correct” or rewrite it so it is true.
| Student’s note | Correct, or your correction |
|---|---|
| 1. Surface area: crushing a solid increases how much of it can dissolve. | |
| 2. Stirring: it carries solute-filled water away from the grains and brings fresh water in, so dissolving is faster. | |
| 3. Temperature: for most solids, hotter water raises both the rate of dissolving and the maximum amount that dissolves. | |
| 4. Gases: hot water holds more dissolved gas, so a warm soft drink stays fizzy for longer. | |
| 5. Saturated solution: stirring it harder will make the extra solid on the bottom dissolve. |
Judge Lena’s test
Scenario: bath salts
Lena wants to know whether particle size affects how fast Epsom salts (a bath salt) dissolve. She drops a 10 g lump into 200 mL of tap water at 18 °C. She tips 10 g of powder into 200 mL of warm water at 40 °C and stirs it. The powder disappears in 20 s; the lump takes 140 s. Lena concludes: “Powder dissolves 7 times faster, so surface area has a huge effect.”
1. Identify two variables Lena should have controlled but did not. Explain why her conclusion about surface area is not valid.
2. Lena repeats the test fairly: both cups at 18 °C, neither stirred. The powder takes 25 s and the lump takes 160 s, and both cups end up clear with no solid left. Explain these results using the ideas of surface area, rate and maximum amount.
Read the data: putting the fizz in
Scenario: a soft drink factory
Soft drink factories chill water to a few degrees above freezing before they pump carbon dioxide into it. Table 1 shows how much carbon dioxide can dissolve in water at different temperatures, with carbon dioxide gas above the water at normal air pressure.
| Water temperature (°C) | 0 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| Carbon dioxide dissolved (g) | 3.4 | 2.3 | 1.7 | 1.3 | 1.0 |
Source: published gas solubility tables, rounded to one decimal place.
3. How many times more carbon dioxide can dissolve in water at 0 °C than at 40 °C? Show your working.
Answer times
4. Describe the trend in Table 1. Then explain, using particles, why the factory chills the water before adding the gas.
5. A classmate says: “Heat always helps things dissolve. A soft drink factory should use hot water for everything, both to dissolve the sugar and to hold the fizz.” Evaluate this claim, and suggest a better plan for the factory.
Find the errors
Line 1: crushing only makes the solid dissolve faster; the maximum amount stays the same (1). Lines 2 and 3 are correct. Line 4: hot water holds less dissolved gas, so a warm soft drink goes flat faster (1). Line 5: stirring cannot make more dissolve in a saturated solution; for most solids only heating raises the maximum (1). [3 marks]
Judge Lena’s test
1. Water temperature (18 °C vs 40 °C) (1) and stirring (only the powder was stirred) (1). She changed three things at once, so the “7 times” includes the effects of heat and stirring, not just particle size (1). [3 marks]
2. The powder has a much larger surface area, so water particles attack more of it at once (1). That makes its rate of dissolving faster, 25 s instead of 160 s (1). Both cups end up clear with the same 10 g dissolved, because surface area changes only the rate, not the maximum amount (1). [3 marks]
Read the data: putting the fizz in
3. 3.4 ÷ 1.0 (1) = 3.4 times (1). [2 marks]
4. As the temperature rises, less carbon dioxide dissolves, from 3.4 g at 0 °C to 1.0 g at 40 °C (1). In warm water the particles move faster and shake gas particles back out of the spaces between them (1). Cold water lets more carbon dioxide stay dissolved, so the drink holds more fizz (1). [3 marks]
Question 5
Marking guide: 1 clear position (partly right: true for sugar, wrong for the gas); 1 heat helps sugar: faster-moving water particles knock sugar loose more quickly; 1 and for most solids hot water raises the maximum amount; 1 heat works against the gas: less carbon dioxide dissolves when warm; 1 data, e.g. 3.4 g at 0 °C falls to 1.0 g at 40 °C; 1 better plan: dissolve the sugar in warm water, then chill the drink before adding the carbon dioxide. [6 marks]
Model answer: The classmate is only partly right. For a solid such as sugar, hot water helps: its faster particles knock sugar loose more quickly, and hot water can hold more sugar. For a gas it is the opposite. Table 1 shows that 3.4 g of carbon dioxide dissolves in 1 L at 0 °C but only 1.0 g at 40 °C, because warm, jiggling water shakes gas particles out. A better plan is to dissolve the sugar in warm water, then chill the drink before pumping in the gas. The drink holds far less sugar than cold water can dissolve, so the sugar stays dissolved.
Common error: “hot water dissolves more of everything”. True for most solids; gases dissolve less in hot water.